The expression of a very large number of genes (mostly glycolytic genes or translational component genes) that influence the growth rate of the budding yeast Saccharomyces cerevisiae is influenced in turn by the action of only a few regulatory proteins. In a 1990 publication my laboratory presented evidence that two of these regulatory proteins, RAP and GCR, activate transcription interdependently. Subsequently, others have predicted the existence of a new class of regulaory molecules, called coactivators, whose postulated role is not to bind DNA but to contact both the DNA-bound activator and the transcriptional machinery. Our idea that GCR may be a coactivator is supported by the following data: GCR is an absolute requirement for activation by a single, isolated RAP binding site (UASPRPG element); GCR-dependent activation in the ADH1 promoter occurs only through the ADH1 UASRPG and an N-terminal potential activation domain is located within one of two separate essential segments of GCR. The putative GCR activation domain is an amphipathic Alpha-helix similar to strong activation domains in GCN4 and VP16, especially with respect to the placement of bulky hydrophobic residues that are known to be important for activation by VP16. The experiments proposed here test the GCR coactivator model and two competing models for GCR/RAP interdependence. The specific goals of this proposal are to: (1) further characterize the essential N-terminal and C-terminal domains within GCR; (2) try to establish the structure/function relationships of those domains; (3) look for qualitative differences between RAP molecules produced in GCR+ and gcr- cells; and (4) test for association between RAP and GCR or an intermediate factor. The successful completion of this research will contribute to our understanding of GCR function, the complex roles of RAP and how those roles are balanced in vivo, and the means by which growth rate is controlled in yeast cells. Understanding the function of RAP and GCR in yeast could simultaneously contribute to our knowledge about how mammalian cells control their growth rate. Finally, if the GCR coactivator model is correct, a great deal should be learned from this work about the mechanism of eucaryotic transcription initiation. This research might therefore ultimately provide information which is of the utmost importance for understanding the complex phenomena of development, cancer, and aging.